Copper oxide particles were produced in an electric field by evaporating copper powder in a mixture gas of Ar (80%) and O 2 (20%) at 13.3kPa. Yellowish particles with Cu 2 O structure and white particles with amorphous structure accumulated on the negative and positive electrodes. Crystallization from the amorphous to the paramelaconite particles was observed at 340°C by in situ observation in a transmission electron microscope. During the cooling process at room temperature, in addition to the crystallized phase, some crystals again became amorphous at 300°C. The particles were also drastically altered in their morphology and structure upon holding the room temperature for more than 10 days. These results have been explained in terms of the difference of reduction upon vacuum heating.